The present invention concerns a method and a means for achieving optimum utilization of the propulsion engine of a vessel, and more specifically, optimum operation of a ship's propeller in relation to an economic utilization of fuel, in relation to the cavitation problem (formation of bubbles in metal) on the propeller surface, as well as in relation to increased manoeuvering safety at maximum utilization of the propeller performance.
This principle can be exploited in different manners, e.g. in connection with ships in regular service between harbours, where it is possible to achieve a more precise timing regarding arrival and quay bookings, since an electronically controlled propeller capacity provides the ability for calculating the precise time of arrival (slot time).
In other cases, it may be desirable from a charterer's view that a closer specified "cost price", i.e. average velocity possibly can be determined.
In time chartering, the freight rate will be determined by speed and consumption, i.e. when tonnage can be regarded as equal in other respects, that tonnage will be preferred which can guarantee the lowest consumption of bunker fuel at a certain stated velocity.
In situations of crisis in the form of risk of running aground or danger of collision with other vessels, it is very important to be able to stop the ship as fast as possible. Lately, high speed sea buses appear in increasingly larger numbers in narrow fiords and closed waters where the traffic of ships and small boats is substantial, and it is therefore important to be able to stop the vessel rapidly.
In all cases, the present invention is based upon an optimum utilization of the action of the single propeller blade during motion in water. Under stormy conditions with high waves, the ship will experience a constantly varying resistance to its motion through the water, sometimes with the propeller more or less freely rotating in the air, and with a subsequent variation of the propeller power. The consequence thereof is an engine load with unefficient utilization of the propeller, with a subsequent reduction in speed and possible cavitation of the propeller blade surface on the lee side.
The optimum utilization of the propeller will therefore be closely connected with the ability of the ship to overcome the water resistance during motion.
Traditionally, most ship's propellers in larger ships are moulded in one piece, without any possibility of turning the attack angle of each propeller blade. Smaller vessels have in many cases variable-pitch propellers, however, the development now shows that increasingly larger ships find advantages in using such propeller types with twistable blades.
The construction of a ship is often made on the basis of a predetermined normal velocity, and it is left to the designer in the shipyard to find the most favourable shape of hull and propeller in order to satisfy such a requirement.
The penetration ability of the hull through the water, or expressed inversely, the resistance to the ship's motion, will vary with draught and load. The attack angle of the propeller or the propeller blade in order to achieve optimum efficiency will therefore also vary, so that a fixed, i.e. not variable-pitch propeller must be chosen using an average consideration. Outside this average, the propeller will not provide optimum efficiency. It is therefore clear that a variable-pitch solution is preferable, however, this has clearly been difficult in cases of large dimensions, partly due to cost savings, partly due to causes connected with technological development.
Parameters influencing the propulsion of a ship in water are draught, wave resistance, induced resistance, wind and weather. Of these parameters, draught and induced resistance are given for one single voyage. (In another voyage, another draught may be present). The other parameters, like wave resistance, wind and weather, will vary all the time.
If one takes as a starting point a situation with a given draught and a given velocity, which may represent an optimum working situation, then both an increase and a decrease of the velocity will imply increased total expenses. In the first case, disproportionate amounts of fuel are used (unlinear relation between fuel consumption and velocity), and in addition, engine wear is increased, and also the risk of propeller blade cavitation, with the consequences of increased costs as to maintenance and repair. In the second case (lowered velocity) the results are prolonged time at sea with increased salary expenses, later time of arrival and the consequences due hereto regarding lower possibility of profits.
A variable-pitch propeller is that part of the solution which may be compared to driving a car with manual gear-shift, but a continuously manual "shifting of gears", e.g. in a storm, would be inconceivable. An automatic mechanism would be preferable, in the form of "measuring force in real time".
The best manner in which to utilize the propeller maximally, is to find that balance point for the attack angle of the propeller blades which results in the best utilization of the applied force.
From Swedish laid-open publications no. 345.634 and 350.938 are previously known methods of controlling the load on ship engines in connection with variable-pitch propellers, where the propeller blade attack angle or "pitch" is controlled in relation to the sensed shaft torque, i.e. sensing of the torque on the propeller shaft, while one attempts to maintain the engine rpm at a constant value. Mainly, these systems relate to an overload protection for the engine, and the main point is filtering and delaying of signals in order to avoid too rapid oscillations when adjusting the propeller pitch.
From Norwegian patent no. 152.968 is previously known a method of regulating the engine of a vessel with a variable-pitch propeller, however, in this case control is only effected in relation to measured values of speed, fuel consumption and rpm. There is no direct measurement of the vessel's driving force.
Also, British patent no. 1.200.588 deals with the control of variable-pitch propellers, however, the parameter sensed in the regulating circuit, is only how large a current is delivered to an electric drive motor.
None of these previous publications go to the core of the matter, namely a direct sensing of the force with which the propeller influences the ship at the present moment.